One important group of catalysts comprises transition metals and, in particular, precious transition metals such as platinum and palladium dispersed on a support member such as a body of ceramic, carbon or the like. Such catalysts can be fabricated to have a very high surface area, particularly if the support member is a high surface area material; furthermore, they maximize the use of precious, catalytic metals such as platinum or palladium. In some instances, catalysts of this type employ alloys of platinum or palladium with other transition metals. Supported catalysts are in widespread use in a number of applications; however, there is still a need to improve and optimize catalysts of this type.
Supported, precious metal catalysts often have problems of stability resulting from loss and/or migration of the metal on the surface of the support. In some instances, metal particles can be removed from the support by mechanical and/or chemical actions thereby decreasing the effectiveness of the catalyst. Also, metal particles can move across the surface of the support and agglomerate thereby reducing the effective surface area of the active metals. All of such occurrences decrease the effectiveness of the catalyst. Also, it is generally difficult to provide a fine dispersion of particles having the appropriate size range; consequently, many such supported catalytic materials utilize an excess of platinum or palladium thereby increasing their cost and decreasing their efficiency.
As will be explained hereinbelow, the present invention provides a supported catalytic material in which particles of platinum and the like are firmly anchored to a support substrate via an at least partially pyrolyzed transition metal macrocycle so as to prevent loss and/or agglomeration. The catalysts of the present invention, among other things, have very good tolerance for methanol, which makes them very useful in fuel cell applications. Furthermore, the method and materials of the present invention provide for a very good control of the ultimate particle size of the catalytic material.
While the use of metal macrocycles in connection with the preparation of catalysts is known in the prior art, such prior art processes differ significantly from those of the present invention, and do not produce the materials of the present invention or achieve the benefits thereof. For example, published patent application US 2004/0058808 discloses a process wherein transition metal macrocycles are utilized to prepare a platinum-free catalyst. Published application US 2004/0010160 discloses a process wherein an iron-containing macrocycle is pyrolyzed onto a carbon support. A noble metal is subsequently deposited onto the prepared support, and in an optional step, noble metal may be alloyed with a promoter metal, which may be another noble metal or a transition metal. The promoter metal is separate and distinct from the transition metal which was bonded to the support in the course of its prior preparation.
Details and advantages of the materials and methods of the present invention will be apparent from the discussion, description and examples which follow.